The weapon on the Sivar was capable of destroying the Goddard colony, but nothing the size of a planet. The Behemoth on the other hand, was designed to blow Kilrah into pieces with a single shot. Blowing Kilrah apart with the Behemoth would have had the same impact that the Temblor bomb attack did - ending the war. Had they just used their version of the Sivar weapon, there most likely would have been a lot of destruction (assuming the ship carrying the weapon wasn't destroyed while recharging after the first shot), but nothing on the scale of what it would take to make the Kilrathi surrender.
Plus, the PTC on the Confederation-class dreadnoughts were based on Confed's research into the Sivar's weapon, and they had a rather nasty tendency to...um...explode.